When we are making a buffer solution by solutions of a weak acid and its salt like $\ce{CH3COOH}$ and $\ce{CH3COONa}$, or by a weak base and its salt like $\ce{NH3}$ and $\ce{NH4Cl}$, we can use the Henderson equation to determine its $\mathrm{pH}$. But, when we are making a buffer solution by two salts of a polyprotic acid like $\ce{Na2HPO4}$ and $\ce{NaH2PO4}$, how can we calculate the resulting $\mathrm{pH}$? Can Henderson's equation still be used? Or, is there any other formula?

For example, the pH of a solution of $\ce{NaHCO3}$ is given by the formula $\frac{1}{2}(\mathrm{p}K_\mathrm{a1} + \mathrm{p}K_\mathrm{a2})$. But what would be the formula if the salt is from a triprotic acid? Again, what will be the formula if the solution is a mixture of salts?

I need a theoretical calculation of the $\mathrm{pH}$, and this is not for any laboratory type experiment.

  • 1
    $\begingroup$ You can use Henderson-Hasselbach. The equations will independently hold for both equilibria. You use use the total amount in solution to compute the actual amounts. $\endgroup$
    – Zhe
    Nov 21, 2016 at 18:25
  • 2
    $\begingroup$ A useful internet search for this type of problem is 'systematic calculation of pH'. These calculations can get messy pretty quickly, so take it slow. $\endgroup$
    – BiggChemT
    Nov 21, 2016 at 21:48
  • $\begingroup$ @Zhe How can I use the equation? $\endgroup$
    – S R Maiti
    Nov 23, 2016 at 9:02
  • $\begingroup$ chemistry.stackexchange.com/questions/8254/… $\endgroup$ Nov 25, 2016 at 6:05


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.